Mainstream media reports are already rife with dire predictions for the upcoming flu season, but experts say it’s really too early to tell how severe it will be.
The severity of the flu season in the Northern Hemisphere typically mirrors that of the Southern Hemisphere. Australia’s flu season just ended and—while not as deadly as in 2017—had more documented cases of flu this season than ever before prompting concerns about the season to come in the north. There were also concerns raised about the efficacy of the vaccine used this year in the Southern Hemisphere.
Influenza expert Scott E. Hensley, PhD, associate professor of microbiology at the University of Pennsylvania co-authored a paper in August suggesting that the egg-adapted H3N2 component of the influenza vaccine used in the Southern Hemisphere was mismatched. The study notes that the vaccine elicited an antibody response in ferrets that was focused on the antigenic site A of hemagglutinin, which would cause a problem with H3N2 viruses that have the antigenic site A substitutions. Still, Hensley says it’s too soon to predict what the flu season will be like in the Northern Hemisphere just yet, or how effective the vaccine will be.
“There is a good chance that the vaccine will be very effective. There is a lot of H3N2 diversity around the world right and it is unclear which of these viruses—if any—will circulate in the Northern Hemisphere this year,” said Scott E. Hensley, PhD, associate professor of microbiology at the University of Pennsylvania. “It may end up being an H1N1 year, and the H1N1 component of the vaccine is very well matched to most circulating H1N1 strains.”
The Centers for Disease Control and Prevention (CDC) updated the flu vaccine in spring 2019, making changes to both the H3N2 and H1N1 influenza A strains, and adding coverage for both lineages of influenza B strains to quadrivalent vaccines in hopes of better matching this season’s vaccine to circulating viruses.
The World Health Organization (WHO) also updated its recommendations for the Northern Hemisphere in September at the close of the Southern Hemipshere’s flu season, with two of the four components—A/south Australia/34/2019 (H3N2)-like virus and the B/Colorado/06/2017-like virus (Victoria lineage)—being recommended as a change from the vaccine that was used for this year’s vaccine in the Southern Hemisphere.
The quadrivalent flu vaccine for the Northern Hemisphere now includes protection against the A/Kansas/14/2017 (H3N2)-like virus, the A/Brisbane/02/2018 (H1N1)pdm09-like virus, the B/Colorado/06/2017-like virus (Victoria lineage), and the B/Phuket/3073/2013-like virus (Yamagata lineage), according to the CDC.
So far, national flu activity in the United States is low, with 2.8 percent of samples testing positive since September. There are pockets of higher activity, with high levels of flu-like illness being reported in Puerto Rico and Louisiana, according to CDC.
Influenza A viruses seem to be predominant so far, making up about 58 percent of the cumulative positive samples for far this season. A third of those were the (H1N1)pdm09 strain and two-thirds were H3N2. Forty-two percent of the positive samples were Influenza B type, with the Victoria lineage making up about 96 percent of those samples and the Yamagata lineage accounting for the remainder. Influenza B viruses tend to circulate predominantly in the southern United States, while the Influenza A viruses are more prominent in the northern states, CDC notes.
In terms of other surveillance outside of confirmed samples, the CDC reports that nationwide in late October, 1.7 percent of patient visits to outpatient healthcare facilities were related to flu-like illness, noting that this is below the national baseline of 2.4 percent.
Severity of this year’s viruses is difficult to predict so early, but 4.9 percent of adult deaths in late October were attributed to pneumonia and influenza, which CDC reports is below the epidemic threshold of 5.7 percent. Two pediatric deaths were also attributed to the flu already this season, with one of those deaths associated with an Influenza A virus of unknown subtype and the other to the Influenza B/Victoria virus.